This invention relates to an angle measuring device such as a theodolite.
Heretofore, in order to measure a very small angle with a theodolite or the like, an electrical signal related to an amount of rotation of the scale board in the theodolite has been phase compared with a reference signal, or the very small angle has been determined from the amplitude of an electrical signal related to an amount of rotation of the scale board.
For instance, Japanese Patent Application Laid-Open No. 64056/1978 describes an improvement on the former method. More specifically, the electrical signal A sin (.omega.t+.phi..sub.1) related to the amount of rotation of the scale board is detected, and the very small angle is obtained from the phase difference .phi..sub.1 between that electrical signal and the reference signal sin .omega.t. In order to practice this method, an accurate sinusoidal track is provided on the circumference of the scale board, and four rectangular optical detecting diodes are employed which are arranged at 90.degree. phase intervals in the period of the sinusoidal track. In addition, in order to detect a very small angle, 4096 radial slit tracks and optical detecting diodes (which are also arranged at 90.degree. phase intervals) are employed with the diodes having masks in the form of a sine wave with a period of 13. Accordingly, in the device disclosed by Japanese Patent Application Laid-Open No. 53-64056, the tracks on the scale board and the masks are intricate and must be manufactured with high accuracy, and the phase relations between these elements must be adjusted strictly.
In one example of the latter method, moire stripes or interference stripes are formed by a scale on the scale board and a stationary scale, and a very small angle is obtained from the amplitude of an electrical sine wave corresponding to these stripes. This method is simple in arrangement, but since the amplitude of the output depends to a large extent not only on the amount of rotation of the scale board but also on the variation of intensity of the light source, the value of an angle can only be measured with low accuracy, and it is therefore necessary to correct the value with respect to the variation of intensity of the illuminating light.